1. Field of the Invention
The present invention relates to an improvement to an electric ignition type initiator devised to heat a resistance element connected between a pair of electrical signal input sections, based on electrical signals applied through the electrical signal input sections, to effect combustion of an explosive powder charge by the heat generated by the resistance element. The invention particularly relates to an electric ignition type initiator used as the igniter in a gas generation apparatus used to instantaneously operate a vehicle safety device such as a seatbelt or airbag.
2. Description of the Prior Art
An automotive safety device such as a seatbelt is provided with rapid retraction means that in the event of a collision or the like functions to instantaneously retract the belt to protect the user. Similarly, an airbag is equipped with rapid deployment inflation means that, in the event of a collision or the like, instantaneously inflates the bag with gas to protect the user by absorbing the shock of the collision.
Seatbelt rapid retraction means and airbag rapid inflation means generally use gas generators that generate gas by igniting a gas generating agent. In the event of a crash, the gas generation agent is ignited by a mechanical igniter triggered by the crash impact, or by an electrical igniter triggered by an electrical signal. The pressure of the burning gases thus produced is utilized to rapidly retract a seatbelt and instantaneously inflate an airbag.
FIG. 6 shows a prior art electric ignition type initiator used as the initiator in such gas generation devices. As shown, the ends of two current conduction pins 1 are connected by a resistance element 2, which is covered by a priming charge 3. The pins 1 are affixed by a glass hermetic or resin plug 4. The peripheral portion of the plug 4 has a metal casing 5 that encloses the resistance element 2 and charge 3. Inside, the space is filled with an ignition charge 6.
The element 2 may be metal, or metal alloy wire, or metal formed in a prescribed shape by etching, or a thin film of metal alloy. When a vehicle having this electric ignition type initiator is involved in a crash or the like, a shock sensor detects the impact and outputs a signal that is supplied to the resistance element via the pins 1, generating Joule heat that heats up the element 2. When the element 2 heats up to a prescribed temperature, the priming charge 3 ignites, setting off the ignition charge 6 and igniting the gas generation agent. The gas thus generated is instantaneously delivered to a seatbelt to effect rapid retraction of the seatbelt, or is used to effect instantaneous inflation of an airbag. The shorter the time between shock sensor detection of the collision impact and activation of the seatbelt or airbag, the more securely the user is protected. At the very least, this requires a system response time in the order of milliseconds. Therefore, with respect to the gas generation apparatus used to operate seatbelt retraction or airbag deployment, the electric ignition initiator needs to have a response time in the order of {fraction (1/10)} millisecond.
Moreover, these days when vehicles use both seatbelts and airbags, it is also desirable to have a seatbelt activated before an airbag. Thus, it is also important for an initiator to function with good operating response characteristics in order to achieve precise control of these devices. In the case of an electric ignition type initiator in which the priming charge ignites when the initiator element is heated to a prescribed temperature, the time it takes for the element to reach that temperature has a major bearing on the initiator response characteristics.
In the case of the above prior art electric ignition type initiator, whether the resistance element is a wire or a film, the resistance is uniform along the whole length of the element, and the rise in temperature effected by the electrical signal input is also uniform along the whole length. That means that before the priming charge can be ignited, enough time is required for the whole length of the resistance element to reach the ignition temperature, which is disadvantageous in terms of device response characteristics. Moreover, the amount of heat radiation given off along the length of the element also has a considerable effect on the response characteristics. Also, environmental concerns make it difficult to use explosives that contain lead, such as lead trinitroresorcinate, which is heat-sensitive. This is forcing the use of explosives which have low sensitivity to heat, leading to a further increase in response time.
Related standards stipulate the resistance of the resistance elements used in gas generation systems employed in vehicle safety devices. Thus, it is not possible to increase the heating value by increasing the overall resistance of the element, since this would not comply with the standards. Even if the resistance value were to be increased, it would not improve the response as much as might be thought, since the element would be radiating more heat, as described above.
In light of these circumstances, an object of the present invention is to provide an electric ignition type initiator having good response to an electrical signal input.
To attain the above object, the present invention provides an electric ignition type initiator that applies an electrical signal via a pair of electrical signal input portions to a resistance element connected across the electrical signal input portions to heat the resistance element to cause the heat from the resistance element to ignite an explosive charge and thereby operate a vehicle safety apparatus, wherein the resistance element is formed by etching a film of nickel-chrome alloy provided on a top surface of a substrate, a first side of the resistance element extends in a straight line between the electrical signal input portions, and a second side is inclined from each of the electrical signal input portions so that a width between the first side and each incline on the second side decreases as distance from a respective electrical signal input portion increases, the resistance element having its minimum sectional area where the inclines meet.
When the element is configured so that a pair of conductive portions provided on the top surface of the substrate are connected across the electrical input portions, conductive portions that are each formed of a metal that is a good conductor, such as gold, for example, will be advantageous with respect to facilitating the task of soldering the connections. Forming the element by affixing a film of nickel-chrome to the top surface of the substrate enables volume production to be effected. It also makes it unnecessary to take the heat resistance of the substrate into consideration, so a cheaper material such as a composite plastic can be used, enabling manufacturing costs to be reduced. It is preferable to use a compound of zirconium and an oxidizing agent(potassium perchlorate, for example) as the explosive charge.
Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and following detailed description of the invention.